Timing Chain Tensioner Replacement? | Ford Explorer Forums - Serious Explorations

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Timing Chain Tensioner Replacement?


November 14, 2013
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City, State
Portland, OR
Year, Model & Trim Level
2002 Ford Explorer XLT
I am finishing up installing a rebuilt 4.0L Engine into my 02' X to replace the old one that seized. After taking a part the old one we found a bunch of broken plastic from what appeared to be the timing chain guide, and found that there was a bent valve/rod in 3rd cylinder which broke the piston. It was very similar to a thread (which I can no longer find) where the engine seized for what amounted to a timing chain tensioner wearing out and not holding the chain or the guide in place which lead to the bent valve/rod and ultimately the engine seizing.

So my question is now that I have new tensioners in place on the rebuilt engine, how many miles should I go before replacing the tensioners? Any suggestions for solutions would be greatly appreciated.


Also, any explanation would be awesome because I'm going through this for the first time and learning a ton from my father in law as I go, but certainly have a lot to learn.

tensioner or guide?

If your vehicle has 4wd there are 4 tensioners in your engine: left and right hydraulic/spring tensioners,

primary (crankshaft to jackshaft) chain tensioner

and balance shaft chain tensioner.

Some members refer to the below as tensioners but I call them cassette guide assemblies.


I used to suggest replacing the left and right hydraulic tensioners every 75K miles but no longer do so because often the new ones are weaker than the old worn ones. In my opinion the engine rarely fails due to a weak hydraulic tensioner. However, one can seize in the extended position and cause the associated guide assembly to break because the chain can't flex.

I think an engine rarely fails due to a broken primary chain tensioner. I was unable to slip the primary chain with the tensioner completely removed from the engine. Broken pieces usually fall to the oil pan and are prevented from damaging the engine by the oil pickup tube screen.

Balance shaft chain tensioners frequently fail but rarely cause engine damage.

In my opinion the primary cause of timing chain related engine failure is a broken left or right cassette guide assembly. Pieces can get jammed between the head and the chain causing the chain to slip resulting in piston to valve collision. Also, the hydraulic tensioner piston does not have enough extension range to compensate for a broken guide and the slack chain can slip resulting in valve damage.

This is super helpful Street. So the piston in my 3rd cylinder is the one that went bad and broken pieces of what looked like the guide were found in the oil pan. What would cause the cassette guide assembly to break? how often do you recommend looking into this? Are there any warning signs/sounds in the engine that would alert you to a potential broken cassette guide assembly?

cold start rattle

A broken cassette usually first shows up as a cold engine start up rattle. Later it worsens until it sounds like marbles rattling around inside the engine and persists after engine warmup: Timing chain rattle resolution process - SOHC V6